1. Field of the Invention
This invention relates to the field of hydraulic control circuits and hydraulic actuators for operating automatic transmissions of motor vehicles. More particularly, this invention pertains to clutch actuating systems for operating a continually variable belt-driven transmission in forward drive and reverse drive.
2. Background of the Invention
A continuously variable belt-driven transmission includes an endless belt driveably engaged with a primary pulley and a secondary pulley. Each pulley is supported rotatably on a shaft that includes an axially displaceable conical sheave, which is moved by a fluid-operated servo to change the radial position of the belt on the pulley in accordance with the commanded drive ratio. This system is provided with a line pressure regulator valve and a transmission ratio control valve.
A torque converter, used to extend the ratio range for acceleration after start up, is locked as vehicle speed increases and the torque converter reaches the coupling point. Forward drive and reverse drive are provided by a pair of gearsets driven by the secondary sheave on the same axis as the conventional differential.
The selection of forward drive and reverse drive is made via mechanical controlled hydraulically activated multiple disc clutches. In the operation of the forward and reverse clutches, it is important that there be no roughness associated with the engagement and that any delay in moving the transmission from forward drive to reverse drive be minimized. Generally, harsh engagements result because the speeds of the driving and driven elements of the clutch are not synchronized and because of the large inertia associated with one of the engaging parts.
To avoid harsh engagement of the clutches in the automatic transmission, orifices located in the line feeding the clutch have been used to restrict the flow rate of fluid to the cylinder of the clutch. If the flow rate is restricted, the engagement of the clutch is smooth because the speed and inertial differences can be accommodated through slipping of the clutch members, but a substantial delay in engaging the clutch producing the required drive necessarily results. To avoid the delay associated with the use of orifices, hydraulic actuating circuits of this type have been developed such that the orifice is included in the circuit that actuates the clutch only after pressure in the clutch piston has risen after the clearances among the clutch plates, piston and load block have been taken up. Generally, in the operation of systems of this type a valve senses this pressure rise and thereafter diverts flow through the orifice at a rate that is sufficiently low to avoid the impact of clutch engagement.
Generally, in systems of this type, engagement impact increases directly with the size of the orifice, but the delay between the command for clutch engagement and the actual engagement increases inversely with the size of the orifice.
Accumulators having a spring-loaded piston reciprocating within a cylinder that is filled as a function of the throttle angle of the engine have been used to cushion the impact of clutch engagement in an accelerating vehicle when the throttle angle increases, but to allow immediate engagement when the throttle angle is low.